The present invention relates to an optical information recording/reproducing apparatus and a method for recording and/or reproducing information in and from an optical information recording medium using holography.
Today, it has become possible to commercialize optical discs having storage densities on the order of 50 GB in consumer applications owing to the Blue-ray Disc™ standards using blue-violet semiconductor lasers. There is a demand for a technique that will achieve optical discs having larger storage capacities of 100 GB to 1 TB comparable with those of hard disc drives (HDDs).
However, in order to achieve such ultrahigh densities with optical discs, a novel higher-density technique different from a high-density technique relying on higher numerical apertures of objective lenses is needed as well as the use of shorter wavelengths.
While researches concerning next-generation storage techniques are being conducted, a holographic recording technique for recording digital information by employing holography has attracted attention.
The holographic recording technique is a technique for recording information in a recording medium by superimposing signal light having information about pages of data modulated in two dimension by a spatial light modulator with reference light within the recording medium and by producing refractive index modulation in the recording medium by the resulting interference fringe pattern.
During reproduction of information, if the recording medium is illuminated with the reference light used during recording, the hologram recorded in the recording medium acts like a diffraction grating and produces diffracted light. This diffracted light is reproduced as the same light as the recorded signal light including phase information.
The reproduced signal light is detected at high speed in a two-dimensional manner using a light detector array such as a CMOS or CCD sensor. In this way, the holographic recording technique makes it possible to record two-dimensional information in an optical recording medium at once using a single hologram and to reproduce the information. In addition, a plurality of pages of data can be written in a position of a recording medium in an overlapping manner. Hence, massive information can be recorded and reproduced at high speed.
One available holographic recording technique is disclosed, for example, in JP-A-2004-272268, where a so-called angle multiplexed recording method is set forth. A signal light flux is condensed onto an optical information recording medium via a lens. At the same time, the medium is illuminated with reference light of a parallel light flux such that the signal light flux interferes with the reference light, thus recording a hologram. A different page of data is displayed on the spatial light modulator while varying the incident angle of the reference light entering the optical recording medium, thus performing multiplexed recordings.
A means for achieving larger data storages by a holographic recording technique is disclosed, for example, in JP-A-2012-27996, where multi-valued phase information is attached to each pixel of signal light.